The viscose rayon method and cupro-ammonia method are well known as methods for producing cellulose filaments or fibers. These conventional methods are disadvantageous in that the solvents for cellulose are difficult to recycle and the discharge of the solvent causes environmental pollution.
In consideration of environmental safety, Japanese Examined Patent Publication No. 60-28848 (which corresponds to U.S. Pat. Nos. 4,246,221 and 4,416,698) discloses a method of producing shaped cellulose articles, for example, filaments or fibers or films, by dissolving cellulose in N-methylmorpholine-N-oxide to provide a dope solution, and then shaping the cellulose dope solution. This method is advantageous in that the solvent for cellulose can be recoved from the dope solution and reused.
Japanese Unexamined Patent Publication No. 4-308220 or the corresponding U.S. Pat. No. 5,252,284 discloses a method of spinning the cellulose solution similar to that mentioned above through a spinning nozzle having a small hole diameter at a low spinning draft to form small denier filaments.
Generally, the solution of cellulose in N-methylmorpholine-N-oxide has a high viscosity and exhibites significant elastomeric behavior. Since this high viscosity solution is difficult to deform, it is difficult to make a large spinning draft ratio (the ratio of the take-up velocity to an extruding velocity of solution through a nozzle orifice).
Also, where the shaping procedure is carried out by using a nozzle having a small orifice diameter or a slit having a small width, an increase in the extrusion rate of the solution results in the generation of melt fractures which means a melt flow instability of the solution through a nozzle orifice. Therefore, the shaping conditions under which the solution can be stably shaped are limited to a range in which melt fractures are not generated, and this limitation reduces productivity.
To conduct the shaping procedure without reducing the productivity, the viscosity of the solution is reduced. Where a low viscosity solution is used, the generation of melt fractures is restricted and thus the shaping stability of the solution is enhanced. Also, since a low viscosity solution can be easily deformed, it is possible to increase the spinning draft ratio and take-up velocity of the filaments. Generally, the viscoelastic behavior of a solution of a polymeric substance is variable depending on the concentration and temperature of the solution and the molecular weight of the polymeric substance in the solution. Generally, to reduce the viscosity of a polymeric substance solution, the concentration of the polymeric substance in the solution is reduced or the molecular weight of the polymeric substance is reduced.
However, the reduction of the concentration of the polymeric substance in the solution is disadvantageously, reduces the productivity of the shaped products, and the load on the system for recovering the solvent is increased. Also, if a polymeric substance having a low molecular weight is used for the shaping solution, the resultant articles have disadvantageously unstable physical characteristics, for example, the mechanical strength and ultimate elongation can easily vary.
U.S. Pat. No. 4,983,730 discloses a method of preparing a cellulose acetate composition having a high water solubility by blending a high molecular weight cellulose acetate with a low molecular weight cellulose having a viscosity of at least 20% below that of the high molecular weight cellulose acetate. The water-soluble cellulose acetate composition is useful for producing shaped articles having a high tensile strength. In this U.S. patent specification, no solution of cellulose, particularly in N-methylmorpholine-N-oxide, is disclosed. Also, since the cellulose acetate is a polymeric substance which is soluble in a relatively large number of solvents, it is possible to reduce the viscosity of the solution thereof. However, since the solution of cellulose in N-methylmorpholine-N-oxide has a high viscosity and exhibits a significant viscoelastic behavior, the deformation of the solution in the shaping procedure is difficult and thus it is difficult to enhance the productivity of the shaped articles from the solution.